1. Field of the Invention
This invention relates to an optical pickup device for reproducing information signals from an optical disc or an magneto-optical disc.
2. Description of the Related Art
A optical disc is known for reproducing information signals, such as an optical disc or a magneto-optical disc. This type of the optical disc is used for recording acoustic signals or data processed in an information processing apparatus, such as a computer, and has a diameter of 120 or 80 mm and a thickness of 1.2 mm. Among different types of the optical disc, a so-called compact disc (CD) is known. Among the different types of the CD, there is such CD termed compact disc recordable (CD-R) or a compact disc rewritable (CD-RW) on which can be written information signals. In the CD, CD-R or CD-RW, the reflecting surface as a signal recording surface is formed at a position of approximately 1.1 mm inwardly from the surface of one of the signal readout surfaces.
Also, with the recent preference to multi-media of information signals, there is a demand for diversification and voluminousness of information signals, such as picture data or acoustic data handled at a time. For meeting this demand, there is known, as an optical disc aimed at minimizing the medium size while realizing high recording density, a laminated disc obtained on bonding two optical discs each having a diameter of 120 mm and a thickness of 0.6 mm to provide an entire thickness of 1.2 mm, or a laminated optical disc obtained on laminating an optical disc 0.6 mm in thickness and a reinforcement disc 0.6 mm in thickness to provide an entire thickness of 1.2 mm. This optical disc is generally termed a digital video disc (DVD). That is, with this DVD, the reflective surface as a signal recording surface is formed at a position of 0.6 mm inwardly from one of the readout surfaces.
As an optical disc which realizes high recording density without narrowing the tolerance due to the disc tilt, there is also proposed an optical disc having a diameter of 120 mm and a thickness of 0.1 mm and a reinforcement disc having a thickness of 1.1 mm to provide an entire thickness of 1.2 mm. This optical disc is herein termed a high recording density disc. With this high recording density disc, the reflective surface, as a signal recording surface, is formed at a position of 0.1 mm inwardly from one of the signal readout surfaces.
As an optical pickup device for reproducing information signals from this high recording density disc, there is proposed such a device provided with a double-lens type objective lens having two lenses having the optical axes coincident with each other, as disclosed in U.S. Pat. No. 5,712,842 or U.S. Pat. No. 5,764,613. This double lens type objective lens is made up of a first lens arranged facing the high recording density disc, and a second lens having its optical axis coincident with the optical axis of the first lens. By the first and second lenses, a numerical aperture NA not less than 0.7 is realized. The first and second lenses are termed herein as a forward lens and a backward lens, respectively.
However, if, with the above-described optical pickup device, information signals are to be reproduced from the high recording density disc, with the numerical aperture NA of the objective lens unit of 0.85, the operating distance between the signal readout surface of the high recording density disc and the forward lens of the objective lens unit, referred to herein as the working distance, is 0.1 mm, with the surface spot diameter on the signal recording surface of the high recording density disc being of the order of 100 .mu.m.
The working distance and the surface spot diameter in case of reproducing the CD or the DVD are not less than 1.2 mm and 1 mm, respectively. As compared to these values, the optical pickup device used for the high recording density disc has a working distance and a surface spot diameter of significantly small values.
Thus, the above-mentioned optical pickup device has a drawback that, if there is contamination, such as dust and dirt of the order of 1 mm or micro-sized flaws on the signal recording surface, focussing servo or tracking servo of the objective lens unit is disengaged.
Therefore, if, with the above-described optical pickup device, the focussing servo of the objective lens unit is disengaged, the movable parts, such as an objective lens unit, collides against the high recording density disc, because the working distance is of the order of 0.1 mm.
With the optical pickup device, it is contemplated to employ a reflection type photosensor or a capacitance detection sensor for controlling the small working distance to prevent the collision of the objective lens unit against the high recording density disc. However, there is raised an inconvenience that the device is poor in operational reliability and that the optical pickup device tends to be increased in size.